Improve specs for repository backups

What does this MR do?

This MR improves the specs for repository backups. This is mostly changes to the spec for Backup::Repositories, with a small change to backup_rake_spec.rb which acts as an integration test.

The changes test that wiki repositories are dumped, and that Backup::Repositories can restore both project and wiki repositories.

This is preparatory work for adding support for design repositories #233430 (closed).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done